The invention relates to a method and a device for determining pressure quantities, in particular, those of a wheel braking pressure and/or an admission pressure in a braking system of a vehicle having an antilock braking system (ABS) and/or traction control system (TCR) and/or electronic stability program (ESP), in accordance with the precharacterizing portions of the independent claims.
A braking pressure control system is known from German Patent Application 40 30 724 A1 in which desired braking pressures are determined for the wheel brake cylinders based on measured values and estimated values. Activation times for a hydraulic system having inlet valves are determined from the desired braking pressures using a hydraulic model. The actual braking pressure in the wheel brake cylinder is estimated from the brake circuit pressure, which is assumed to correspond to the admission pressure of the braking system measured by a sensor, and the activation times of the valves assigned to the individual brakes using a hydraulic model. Moreover, it is necessary to use pressure sensors in the braking system, an admission pressure sensor, in particular.
German Patent Application 43 40 921 A1 subsequently shows a concrete embodiment in the form of a special mathematical relation for estimating the brake circuit pressure for each brake circuit. In doing so, higher brake circuit pressures than can be produced by the driver are obtained by using a precharging pump. In turn, the concrete model calculation used to determine the desired braking pressure necessarily uses the estimated braking pressures and the admission pressure measured again by a sensor. This does in fact eliminate the use of additional sensors for pressure measurement; however, this method requires the use of the admission pressure as a measured value.
A method and a device to replace sensors for detecting a pressure quantity is described in German Patent Application 195 28 697 A1. In doing so, a differential pressure between a first pressure in a first line and a second pressure in a second line is determined in a braking system having an antilock braking system or a traction control system with a feed pump. A quantity serving as a measure of the speed of the feed pump is used as the starting point. A voltage electromagnetically induced by the overtravel of the feed pump is used for this purpose. Thus the dependence on the electromagnetically induced voltage is only present when the associated pressure medium is delivered via the pump; otherwise the pressure quantity is not determined or specified. In addition, the determined value is used as a measure of the rotational speed, the electromagnetically induced voltage, in particular. As a result, the rotational speed obtained from it is strongly dependent on voltage and temperature. These dependencies are not taken into account or compensated. Also, the influence of the production variation must not be ignored in the speed-forming quantity. In addition, with this method, pressure sensors can only be eliminated in the determination of a differential pressure between a first line and a second line. To determine an absolute pressure, it is again necessary for a pressure quantity to be detected as a reference by a sensor.
Thus it is evident that the known methods and devices named in the related art cannot provide optimum results in every respect.
The foregoing gives rise to the object to determine a pressure quantity in a braking system, an admission pressure and/or a wheel brake pressure, in particular, as an absolute value without the use of pressure sensors. In doing so, it is possible to use quantities which are already available in the particular control systems such as an antilock braking system (ABS), traction control system (TCR) an electronic stability program (ESP), a brake booster (HBV, HBU), a cruise control system (ACC), hill holder control (HHC) or hill descent control (HDC) without having to explicitly detect pressure quantities. Accordingly, the methods and the particular corresponding device for increasing the driving stability and/or safety in vehicles can be used for all systems or controls that influence the braking effect.
Using the methods according to the present invention in the device according to the invention, in particular, makes it possible to eliminate the use of pressure sensors. In particular, it is possible to eliminate the admission pressure sensor system used in the above-cited known methods. The values required, such as the starting value and the admission pressure value in estimating the wheel braking pressure or wheel pressure, can be determined using available signals such as wheel velocity, rotational speed or voltage or power consumption of the pump motor.
The idea of pressure estimation, the determination of the admission pressure, in particular, lies in the fact that the required torque of the pump motor in return delivery is directly proportional to the admission pressure. The disadvantage of pressure detection as a function of rotational speed is compensated by the use of power consumption as a measured value. This is possible since the power consumption of the pump motor is a function of the associated torque. Accordingly, the power consumption of the pump motor increases in linear proportion to the torque. That which, for example, the electromagnetically induced overtravel voltage, the current signal is continuously available for analysis and not only when the pump is in operation.
The voltage and/or temperature dependence of a value used as a measure of rotational speed can be eliminated or compensated by the use of currents, of at least one motor current, in particular. Also, the influence of manufacturing variance is then of less significance than in the rotational speed-forming value such as the electromagnetically induced motor voltage or overtravel voltage.
It is additionally advantageous to include a pressure-dependent factor in a relation between pressure reduction time and the volume of the pressure medium in question, in a pressure accumulator, in particular, for the pressure estimation since this takes into account the dependence of this relation or of this interrelationship on the pressure level and accordingly on the further increase of the quality and safety of the methods and device.
Advantageously, the influence of temperature on the precision of estimation of the particular, at least one, pressure quantity, of the admission pressure, in particular, is compensated according to the present invention in such a way that the temperature information, for example, is included in a proportionality factor for determining the pressure quantity or a corresponding mean value of proportionality factors and the latter are thereby varied and adapted.
A possible temperature dependence of the estimation, of the interrelationship between pressure reduction time and the volume cited in a preceding paragraph, in particular, is taken into account by applying a similarity or comparability of the temperature dependence of a current and a viscosity of the pressure medium and then including them in the method and device according to the invention. Moreover, an assumed and specified dependence of a current on the viscosity of the pressure medium advantageously even allows a temperature and estimation error to be compensated by one-time, constant, or continuous, variable inclusion of this current in the methods and particular device, for example, by forming a minimum in the current curve.
A further advantage is the fact that the methods of the invention and the particular corresponding device allow a plausibility check of the determined pressure variable or pressure variables to be performed, resulting in both a very high quality of the estimate or determination of the value and an increase of the reliability of the systems and controls influencing the braking effect by the resulting monitoring functions for the value.
These advantages apply as well to the case of xcexc-split braking, i.e., an influencing of the braking effect in which at least one of the wheels is on a roadway surface having a coefficient of friction xcexc which differs from the others. This results in an additional increase in the reliability and calculability of the vehicle behavior.
An additional great advantage is the use of the methods and/or parts thereof already in the first control cycle in the systems and controls influencing the braking effect. As a result, the complete reliability of the system is present from the start. Specifying a higher coefficient of friction than the actual one increases the reliability even further.